Metal strip may be heat treated as an endless belt passing horizontally or vertically (looping tower) through a furnace after which the strip is rewound as a coil. Alternatively, the strip may be heat treated in a batch furnace with the strip tightly wound as coils vertically stacked on edge, one on top of the other.
Batch coil annealing furnaces (sometimes called "box annealing furnaces" or "bell shaped furnaces") have been long used and are well known in the industry. Batch coil annealing furnaces include a base or an annealing stand upon which the steel coils are stacked vertically, edge upon edge, and over which a removable inner cover is placed. It is to be appreciated that the metal strip or coil is formed by winding the strips or sheets into coils having an axial passage bounded by the inner diameter of the winding. Several of the coils are stacked edge to edge and separated axially by a diffuser plate positioned in between adjacent coils. An outer cover, in turn, is placed over the inner cover. The outer cover along with the inner cover and the base comprise the three major components of the furnace. Both covers are removably sealed to the base and the outer cover typically contains gas fired burners for heating the inner cover. The inner cover, in turn, transfers heat to the coils. The primary mode of heat transfer from the cover to the coils is by radiation. Additionally, a defined furnace atmosphere is circulated within the inner cover to achieve more rapid and uniform heat transfer by convection while maintaining a desired gas composition for metallurgical process purposes. Batch coil annealing processes in the steel mill industry typically take anywhere from about twenty hours to as long as several (3) days to complete.
More specifically, coils are stacked coaxially upon one another within the inner cover with the axial passage of each coil aligned to form central, axial path. A radial fan in the base of the annealing furnace in alignment with the axial path draws furnace atmosphere within the inner cover down through the axial path into the annealing stand or base. A diffuser plate within the base then directs or causes the inner cover atmosphere to travel through the base and vertically upward back into the inner cover at a position within the annular space between the inner cover and the outside diameter of the coiled strip. The atmosphere then travels up through a top space between the top of the coils and the top of the inner cover and back down to the fan through the axial path in the center of the stack of coils.
Even with the use of a recirculating fan, there is nonuniform heat transfer to the work and the rate of heating to achieve annealing is limited. The atmosphere heats (from the hot wall of the inner cover) as it rises in the annular space between the coils and inner cover and is hottest when it reaches the top of the coils. The top outside corner of the top coil is exposed to the radiant energy from the side and the top of the inner cover and is the hottest spot in the stack of coils. On the other hand, the lower most corner of the lowest coil is, in contrast, the coldest spot during heating of the work. This results in a temperature differential which in turn limits the rate at which the work is heated and the rate at which uniformity of the temperature within the coils can be achieved. The problem is further compounded by the fact that the upper coils in the stack are usually the smaller and lightest coils.
Turning next to the cooling part of the process, it has been known in the past to provide base cooling. Base cooling has been achieved using either internal heat exchangers or external heat exchangers. The internal heat exchangers are conventionally supplied under the brand named INTPAK00L.TM. and basically comprise bare or ribbed tubes which are installed as round coil in the base. The tubing is prone to fail because it is suddenly subjected to cold water which leads to localized boiling. Subjecting the internal coils to numerous heating-cooling cycles causes thermal fatigue as well as thermal shock. The end result is a break of the heat exchanging tubing after being exposed to a finite number of thermal stress cycles with the result that there will then be a lost load of steel as a result of the water normally intended to be circulated within the coils, being converted to steam and oxidizing the work. One of the patents incorporated by reference herein, Mayers U.S. Pat. No. 4,275,569, is directed to the concept of extending the life of the internal heat exchangers. External heat exchangers suffer from another problem. They require considerable space which is usually at a premium in the basement of a typical annealing facility. Also external heat exchangers normally fail at the connections between heat exchanger and base. Again, thermal fatigue is the cause of the failure. Cracks normally lead to loss of atmosphere and leaking oxygen with the result that when the failure occurs, oxidation and a lost work load will also result.
Apart from the heat distribution aspects of the invention, inner covers for batch coil annealing furnaces are made from heat resistant alloy such as 309 stainless to withstand relatively high temperatures (although not "high" in the furnace sense) and repeated heating and cooling under production conditions. However, inner covers tend to fail on a regular basis. Failure occurs in one of two general locations. Failures occur in the proximity of the burners where flame gases directly impinge on the outer surface of the inner cover. The other major failure location is in the frustoconical section between the vertical cylinder section and horizontal bottom flange of the inner cover. Failures in both cases occur because of crack formation i.e., fatigue cracks. When failure occurs, there is increased leakage of the protective atmosphere gas from the inner cover. In the case of HNX gas such leakage is tolerable as long as the crack is small. However, in case of high hydrogen content atmospheres, the cracks result in a larger loss of atmosphere due to the lower density of atmosphere gas and will eventually create an unsafe condition when pure hydrogen escapes through a large crack and mixes with air. Numerous efforts have been made to eliminate or reduce the frequency of the failures. More expensive alloys have been used. Radiation shields have been added to the inner cover in the vicinity of the burners. Corrugated inner covers have been tried. Other attempts to address this problem have included different welding techniques as well as modified flange cooling or heating approaches. Despite these efforts, inner covers continue to fail on a regular basis.